Anti-nuclear activists have been popping Champagne corks over the news that the last remaining nuclear power plant in California, the Diablo Canyon facility outside of San Luis Obispo, is expected to shut down within the next nine years.

But a number of critics say if history is a guide, California will become more dependent on a fossil fuel — natural gas — in order to make up for the electricity generation that will be lost when Diablo Canyon gets mothballed.

"What is absolutely outrageous is to suggest you can take Diablo Canyon offline and not increase our dependence on methane, fracked gas and not increase carbon emissions,” said Michael Shellenberger, president of Environmental Progress, a pro-nuclear environmental group based in Berkeley.

Executives at Pacific Gas & Electric, the utility that runs Diablo Canyon, say the 2,160 megawatts of electricitythe plant generates can be offset by energy storage, greater efficiency and renewable power from sources such as solar and wind.

The president of PG&E's electricity division, Geisha Williams, said the closure of the plant's two reactors — scheduled in November 2024 and August 2025 — will not translate into a spike in natural gas usage in the state.

"We've got roughly 10 years between now and then," Williams said. "Our approach will be to have a highly integrated portfolio of renewable and other sources so we don't have to increase the gas usage. That's really our focus and that's really our goal."

Within days of PG&E's announcement of its intention to close the plant, two studies were released claiming natural gas use will go up as Diablo goes down.

An analysis by the PIRA Energy Group, based in New York City, predicted that a downtrend for natural gas use in Northern California would reverse after Diablo Canyon shut down. Instead, natural gas would rise to 34 percent from 2023 to 2026.

"That's a lot of energy supply that's being taken off the system that needs to be replaced," said Morris Greenberg, PIRA's managing director of North American Power. "And it really isn't cost effective to replace all that with renewables over a two-year period. As a result, you need more natural gas."

“Gas-power plants will probably be needed for backup when wind and solar plants aren’t available,” researchers Rob Barnett and Kit Konolige wrote in their analysis June 22.

Todd Strauss, PG&E’s senior director of energy policy planning and analysis, said the Bloomberg report does not reflect the numbers that PG&E forecasts for a rapidly changing energy sector.

"It's a very different world going forward," Strauss said.

Pointing to growth in renewables, improvements in energy efficiency and storage capability, PG&E officials think they won't have to replace every megawatt of electricity generated at Diablo to meet customer demand in the future.

Plus, more improvements in energy efficiency measures leads PG&E to expect that demand will fall by 2,000 gigawatt-hours by 2025. It takes about 6,000 gigawatt-hours to power 1 million homes for a year in California.

"We're not replacing every megawatt hour because we're shaping the energy," Strauss said, "and we're using energy efficiency and renewables."

PG&E's proposal to shut down Diablo Canyon came with the backing of a number of environmental organizations, including Friends of the Earth, the Natural Resources Defense Council and Environment California, who signed off on the deal June 21.

Solar and wind energy are plagued by intermittency — that is, the ability to generate power when the sun isn't shining or the wind isn't blowing — and utilities across the country have increasingly turned to natural gas as their source for consistent, baseload power.

Largely due to the practice of hydraulic fracturing — the process that sends highly-pressurized liquids into the ground to break through rock formations — natural gas is abundant, which makes it inexpensive, at least in the past seven years.

In 2011, the California Energy Commission (CEC) said 18.2 percent of the state's power generation came from nuclear energy.

But when the San Onofre Nuclear Generating Station closed in January 2012, nuclear's share of in-state power generation immediately dropped to 9.3 percent.

Shellenberger's group estimates a Diablo shutdown will increase the share of natural gas in California's energy mix to about 70 percent.

"That’s an extremely high and dangerous level of dependence on any fuel, much less one that, when you look at 40 years of natural gas prices, is famous for having price spikes that can cause significant problems to the economy and significant energy shortages," Shellenberger said.

But Strauss said that unlike San Onofre, which shut down quickly after leaking steam generator tubes were discovered, Diablo Canyon is not scheduled to close for another nine years.

"With enough planning we'll be able to get the right resources in place," Strauss said. "So given that opportunity, given that portfolio, we actually see it's not a matter of adding more gas to replace Diablo."

Steve Berberich, CEO for the California Independent System Operator, the nonprofit that manages the state's electric grid, thinks PG&E can meet its goals without a spike in natural gas.

"Between a mix of geothermal and solar and wind, if we get the right portfolio I think we can," Berberich said.

PG&E officials, in a conference call Thursday with the Union-Tribune, said the decision to retire the two reactors at Diablo Canyon was largely driven by state policy.

In the latest iteration of California's move to cleaner energy sources, Senate Bill 350 requires the state to get 50 percent of its energy from renewable sources by 2030 and double energy efficiency in homes and commercial buildings.

The California Public Utilities Commission (CPUC) has also ordered the state’s big three investor-owned utilities to add 1.3 gigawatts of energy storage to their grids by the end of the decade.

"This is a reflection of the changing energy landscape, being responsive to changing energy policies and mandates that we have to meet," PG&E spokesman Blair Jones said.

The Diablo Canyon shutdown is not a done deal. It's contingent on a number of regulatory actions, including approvals from the CPUC.

PG&E will file the joint proposal to close Diablo Canyon later this month before the CPUC in which more details are expected to be laid out.

Shellenberger said his group, Environmental Progress, will appeal to CPUC commissioners to keep Diablo Canyon online and has not ruled out filing a lawsuit to keep the plant from closing.

"The long-term asset (Diablo Canyon) that this company is proposing to throw away, it’s just astonishing, it’s breathtaking," Shellenberger said. "It’s the crown jewel of PG&E power assets."

Jacobson said he has no doubts that a mixture of renewables, storage and efficiency can replace Diablo's output of low-cost electricity.

"It's not as if we're waiting (for some) invention to come along," Jacobson said. "We have the tools we need to get to 100 percent renewables right now."